Semiconductor structure and method for manufacturing the same

ABSTRACT

A semiconductor structure and a method for manufacturing the same are disclosed. The semiconductor structure includes a semiconductor substrate, a multi-layer stack, a switch device, and an air void. The multi-layer stack is buried in the semiconductor substrate. The multi-layer stack includes a first filling layer and a second filling layer under the first filling layer, the first filling layer has a first etching rate, the second filling layer has a second etching rate, and the first etching rate and the second etching rate are different. The switch device is disposed over the semiconductor substrate. The air void is formed in the multi-layer stack and under the switch device. The air void is surrounded by dielectric filling material.

TECHNICAL FIELD

The present disclosure relates to a semiconductor structure and a methodfor manufacturing the same, and more particularly, to a semiconductorstructure having different filling layers with different etching ratesembedded in a semiconductor substrate and a method for manufacturing thesame.

DISCUSSION OF THE BACKGROUND

Semiconductor devices are essential for many modern applications. Withthe advancement of electronic technology, semiconductor devices aresteadily becoming smaller and smaller while providing greaterfunctionality and including greater amounts of integrated circuits. Dueto the miniaturized scale of semiconductor devices, a conventionalsemiconductor structure including a silicon on insulator (SOI) structureis provided to increase the isolation effect, so that the semiconductorstructure can reduce the parasitic capacitance effect and increase theswitching speed.

The conventional semiconductor structure includes a semiconductorsubstrate, an insulator layer buried in the substrate, and a switchdevice disposed over the insulator layer and the semiconductorsubstrate.

However, as semiconductor devices become smaller and smaller, thecapacitance remaining in the semiconductor device may result incapacitance disturbance and reduced switching speed.

This Discussion of the Background section is provided for backgroundinformation only. The statements in this Discussion of the Backgroundare not an admission that the subject matter disclosed in this sectionconstitutes prior art to the present disclosure, and no part of thisDiscussion of the Background section may be used as an admission thatany part of this application, including this Discussion of theBackground section, constitutes prior art to the present disclosure.

SUMMARY

One aspect of the present disclosure provides a semiconductor structure.The semiconductor structure includes a semiconductor substrate, amulti-layer stack, a switch device, and an air void. The multi-layerstack is buried in the semiconductor substrate. The multi-layer stackincludes a first filling layer and a second filling layer under thefirst filling layer, the first filling layer has a first etching rate,the second filling layer has a second etching rate, and the firstetching rate and the second etching rate are different. The switchdevice is disposed over the semiconductor substrate. The air void isformed in the multi-layer stack and under the switch device. The airvoid is surrounded by dielectric filling material.

In some embodiments, the second etching rate is greater than the firstetching rate.

In some embodiments, the multi-layer stack further includes a thirdfilling layer disposed beneath the second filling layer, wherein thethird filling layer has a third etching rate different from the secondetching rate.

In some embodiments, the second etching rate is greater than the firstetching rate and the third etching rate.

In some embodiments, the air void is formed in the location of thesecond filling layer.

In some embodiments, the semiconductor substrate includes an underlyingsubstrate and an epitaxial semiconductor layer disposed over theunderlying substrate, wherein the multi-layer stack is buried in theunderlying substrate, and the epitaxial semiconductor layer is disposedbetween the multi-layer stack and the switch device.

In some embodiments, the semiconductor structure further comprises awell portion embedded in the epitaxial semiconductor layer, wherein thewell portion is disposed under the switch device and over the air void.

In some embodiments, the semiconductor structure further comprises anetch stop layer disposed between the epitaxial semiconductor layer andthe multi-layer stack.

In some embodiments, the semiconductor structure further comprises: twoshallow trench isolation portions disposed in the semiconductorsubstrate, wherein the switch device is disposed laterally between theshallow trench isolation portions; and two deep trench isolationportions disposed in the semiconductor substrate and respectively underthe shallow trench isolation portions, wherein the multi-layer stack isdisposed laterally between the deep trench isolation portions.

In some embodiments, the semiconductor structure further comprises adielectric layer disposed over the semiconductor substrate andsurrounding the switch device.

Another aspect of the present disclosure provides a method formanufacturing a semiconductor structure. The method includes forming amulti-layer stack in a semiconductor substrate, wherein the multi-layerstack has a first filling layer and a second filling layer under thefirst filling layer, the first filling layer has a first etching rate,the second filling layer has a second etching rate, the first etchingrate and the second etching rate are different, the semiconductorsubstrate has two through vias, and two top portions of the multi-layerstack are respectively exposed through the two through vias; recessingthe multi-layer stack from the two through vias to respectively form twoblind holes in the first filling layer and the second filling layer;selectively etching the second filling layer to form a global cavitybetween the two blind holes; filling the global cavity and the two blindholes with dielectric filling material to form an air void in themulti-layer stack; and forming a switch device over the semiconductorsubstrate, wherein the air void is formed under the switch device.

In some embodiments, the method further comprises: forming a thirdfilling layer of the multi-layer stack in the semiconductor substratebeneath the second filling layer, wherein the third filling layer has athird etching rate different from the second etching rate.

In some embodiments, the air void is formed in the location of thesecond filling layer and surrounded by dielectric filling material.

In some embodiments, the method further comprises: forming an epitaxialsemiconductor layer of the semiconductor substrate over an underlyingsubstrate of the semiconductor substrate, wherein the multi-layer stackis formed in the underlying substrate, and the epitaxial semiconductorlayer is formed between the multi-layer stack and the switch device.

In some embodiments, the method further comprises: forming a wellportion embedded in the epitaxial semiconductor layer, wherein the wellportion is formed under the switch device and over the air void.

In some embodiments, the method further comprises: forming an etch stoplayer between the epitaxial semiconductor layer and the multi-layerstack.

In some embodiments, the method further comprises: forming two deeptrench isolation portions in the semiconductor substrate, wherein themulti-layer stack is formed laterally between the deep trench isolationportions; and forming two shallow trench isolation portions in thesemiconductor substrate and respectively above the deep trench isolationportions, wherein the switch device is formed laterally between theshallow trench isolation portions.

In some embodiments, the method further comprises: forming a dielectriclayer over the semiconductor substrate and surrounding the switchdevice.

In some embodiments, the method further comprises: providing anunderlying substrate of the semiconductor substrate before forming amulti-layer stack in the semiconductor substrate, wherein the underlyingsubstrate has the two through vias; and forming two local trenchesrespectively under the two through vias.

In some embodiments, the method further comprises: forming a globaltrench between the two local trenches in the underlying substrate of thesemiconductor substrate; and filling the global trench and the two localtrenches to form the multi-layer stack in the underlying substrate ofthe semiconductor substrate.

With the design of the semiconductor structure, the semiconductorstructure has an air void formed in the semiconductor substrate. The airvoid can provide the switch device with an improved isolation effect,helping the semiconductor structure reduce capacitance disturbance andincreasing the switching speed of the switch device above the air void.

In addition, the semiconductor structure has multiple filling layershaving different etching rates, which can facilitate the properformation of the air void in the semiconductor structure.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter, and form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the concept andspecific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flow diagram illustrating a method for manufacturing asemiconductor structure in accordance with some embodiments of thepresent disclosure.

FIGS. 2 to 7 are schematic views illustrating stages of manufacturing asemiconductor structure by the method of FIG. 1 in accordance with someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a flow diagram illustrating a method for manufacturing asemiconductor structure 200 in accordance with some embodiments of thepresent disclosure. FIGS. 2 to 7 are schematic views illustrating stagesof manufacturing the semiconductor structure 200 by the method 100 ofFIG. 1 in accordance with some embodiments of the present disclosure. Insome embodiments, the method 100 of FIG. 1 for manufacturing thesemiconductor structure 200 includes a number of operations (S101, S103,S105, S107, S109, S111, S113, S115, S117, S119, S121, S123, S125 andS127), and the description and illustration below are not deemed as alimitation as the sequence of the operations.

In operation S101, as shown in FIG. 2, an underlying substrate 212 of asemiconductor substrate 210 is provided. The underlying substrate 212has two through vias 214. In some embodiments, a masking layer 261 isformed over the underlying substrate 212. The through vias 214 areformed through the masking layer 261 and extend into the underlyingsubstrate 212.

In some embodiments, the masking layer 261 may be deposited by aconventional deposition process, for example, a chemical vapordeposition (CVD) process. In some embodiments, the through vias 214 areformed through an anisotropic etching process to remove portions belowthe through vias 214, for example, a wet etching process.

In some embodiments, the masking layer 261 may be a hard mask includingnitride material, oxide material or other masking material. In someembodiments, the underlying substrate 212 may be formed from a substratesuch as a silicon substrate, an epitaxial substrate, a silicon carbidesubstrate, a silicon-on-insulator (SOI) substrate or the like. In someembodiments, the through vias 214 may be lined with a nitride, anoxide-based material, or another masking material.

In operation S103, as shown in FIG. 2, an etch stop layer 220 is formedover the underlying substrate 212. In some embodiments, the etch stoplayer 220 may be deposited using any conventional deposition method, forexample, a chemical vapor deposition (CVD) process. In some embodiments,the etch stop layer 220 may include silicon carbide (SiC) or silicongermanium (SiGe).

In operation S105, as shown in FIG. 2, an epitaxial semiconductor layer216 of the semiconductor substrate 210 is formed over the underlyingsubstrate 212 of the semiconductor substrate 210. In some embodiments,the epitaxial semiconductor layer 216 is formed on the etch stop layer220.

In some embodiments, the etch stop layer 220 may be formed using anyconventional deposition method, for example, a chemical vapor deposition(CVD) process. The epitaxial semiconductor layer 216 is grown on theetch stop layer 220. In this way, the etch stop layer 220 is formedbetween the underlying substrate 212 and the epitaxial semiconductorlayer 216.

In operation S107, as shown in FIG. 2, a well portion 218 is formed andembedded in the epitaxial semiconductor layer 216 after forming theepitaxial semiconductor layer 216 of the semiconductor substrate 210. Insome embodiments, the well portion 218 may be formed in the epitaxialsemiconductor layer 216 using a conventional ion implantation process, adoping process or other suitable well forming process.

In some embodiments, the well portion 218 may be either a p-doped wellportion or an n-doped well portion. Whether the well portion 218 is thep-doped well portion or the n-doped well portion depends on the type ofthe semiconductor, for example, an NFET or a PFET.

In operation S109, as shown in FIG. 2, two deep trench isolationportions 244 are formed in the semiconductor substrate 210. Morespecifically, the two deep trench isolation portions 244 are formed inthe underlying substrate 212. The two deep trench isolation portions 244are formed at opposite sides of the well portion 218 to provideisolation from well charges producing harmonic distortion.

In some embodiments, the two deep trench isolation portions 244 can beformed using a conventional etching process for deep trenches, forexample, a conventional lithography process, followed by a depositionprocess for filling the deep trenches with oxide filling material, forexample, a chemical vapor deposition or a physical vapor depositionprocess.

In operation S111, as shown in FIG. 2, two shallow trench isolationportions 242 are formed in the semiconductor substrate 210 andrespectively above the deep trench isolation portions 244. In someembodiments, the two shallow trench isolation portions 242 are formedseparate from the epitaxial semiconductor layer 216 and extend into theunderlying substrate 212.

In some embodiments, the two shallow trench isolation portions 242 maybe formed using conventional lithography, etching and depositionprocesses. In some embodiments, the shallow trench isolation portions242 include an oxide material or other suitable insulating material.

In operation S113, as shown in FIG. 2, two local trenches 252 are formedunder the two through vias 214, respectively. In some embodiments, thetwo local trenches 252 are formed separate from the etch stop layer 220and extend into the underlying substrate 212. In some embodiments, thetwo local trenches 252 may be formed using conventional etchingprocesses, such as wet etching processes or the like.

In operation S115, as shown in FIG. 3, a global trench 254 is formedbetween the two local trenches 252 in the underlying substrate 212 ofthe semiconductor substrate 210. In some embodiments, the global trench254 may be formed using conventional etching processes, such as wetetching processes or the like.

In operation S117, as shown in FIG. 4, a multi-layer stack 230 is formedin the semiconductor substrate 210. In some embodiments, the globaltrench 254 is filled with multiple filling materials with differentetching rates to form the multi-layer stack 230. In this way, themulti-layer stack 230 has a first filling layer 232 and a second fillinglayer 234 under the first filling layer 232. In some embodiments, athird filling layer 236 of the multi-layer stack 230 is formed in thesemiconductor substrate 210 beneath the second filling layer 234.

In some embodiments, as shown in FIG. 4, the multi-layer stack 230 isformed in the underlying substrate 212. The multi-layer stack 230 isformed laterally between the deep trench isolation portions 244. Theetch stop layer 220 is formed between the epitaxial semiconductor layer216 and the multi-layer stack 230. Two top portions of the multi-layerstack 230 are respectively exposed through the two through vias 214.

As shown in FIG. 4, the first filling layer 232 has a first etchingrate, the second filling layer 234 has a second etching rate, and thefirst etching rate and the second etching rate are different. In someembodiments, the third filling layer 236 has a third etching ratedifferent from the second etching rate. In some embodiments, the firstetching rate and the third etching rate are the same. In someembodiments, the second etching rate is greater than the first etchingrate and the third etching rate. In some embodiments, the first fillinglayer 232, the second filling layer 234 and the third filling layer 236have different materials or different densities.

In operation S119, as shown in FIG. 5, the multi-layer stack 230 isrecessed from the two through vias 214 to respectively form two blindholes 256 in the first filling layer 232 and the second filling layer234. In some embodiments, the two blind holes 256 are formed through thefirst filling layer 232 and the second filling layer 234 into the thirdfilling layer 236. In some embodiments, the multi-layer stack 230 may beformed using conventional etching processes, such as a wet etchingprocesses or other suitable anisotropic etching processes.

In operation S121, as shown in FIG. 6, the second filling layer 234 isselectively etched to form a global cavity 258 between the two blindholes 256 after forming the blind holes 256 in the first filling layer232, the second filling layer 234 and the third filling layer 236.

In some embodiments, the second etching rate of the second filling layer234 is greater than the first etching rate of the first filling layer232 and the third etching rate of the third filling layer 236, andtherefore the second filling layer 234 is etched more quickly than otherfilling layers to form the global cavity 258 first. In some embodiments,the global cavity 258 may be formed using conventional etchingprocesses, such as a wet etching process or other anisotropic etchingprocesses.

In operation S123, as shown in FIG. 7, the global cavity 258 and the twoblind holes 256 are filled with dielectric filling material 240 so as toform an air void 238 in the multi-layer stack 230. The air void 238 isformed in the location of the second filling layer 234. In someembodiments, the air void 238 is surrounded by the dielectric fillingmaterial 240.

In operation S125, as shown in FIG. 7, a switch device 260 is formedover the semiconductor substrate 210, wherein the air void 238 is formedunder the switch device 260. The epitaxial semiconductor layer 216 isformed between the multi-layer stack 230 and the switch device 260. Thewell portion 218 is formed under the switch device 260 and over the airvoid 238. The switch device 260 is formed laterally between the shallowtrench isolation portions 242.

In operation S127, as shown in FIG. 7, a dielectric layer 262 is formedover the semiconductor substrate 210. The switch device 260 issurrounded by the dielectric layer 262.

As shown in FIG. 7, the semiconductor structure 200 is provided. Thesemiconductor structure includes the semiconductor substrate 210, themulti-layer stack 230, the switch device 260, and the air void 238. Themulti-layer stack 230 is buried in the semiconductor substrate 210. Themulti-layer stack 230 includes the first filling layer 232 and thesecond filling layer 234 under the first filling layer 232, the firstfilling layer 232 has the first etching rate, the second filling layer234 has the second etching rate, and the first etching rate and thesecond etching rate are different. The switch device 260 is disposedover the semiconductor substrate 210. The air void 238 is formed in themulti-layer stack 230 and under the switch device 260. The air void 238is surrounded by the dielectric filling material 240. The multi-layerstack 230 further includes the third filling layer 236 disposed beneaththe second filling layer 234. The third filling layer 236 has a thirdetching rate different from the second etching rate. The second etchingrate is greater than the first etching rate and the third etching rate.The air void 238 is formed in the location of the second filling layer234.

As shown in FIG. 7, the second etching rate is greater than the firstetching rate. The semiconductor substrate 210 includes the underlyingsubstrate 212 and the epitaxial semiconductor layer 216 disposed overthe underlying substrate 212. The multi-layer stack 230 is buried in theunderlying substrate 212, and the epitaxial semiconductor layer 216 isdisposed between the multi-layer stack 230 and the switch device 260.

As shown in FIG. 7, the well portion 218 is embedded in the epitaxialsemiconductor layer 216. The well portion 218 is disposed under theswitch device 260 and over the air void 238. The etch stop layer 220 isdisposed between the epitaxial semiconductor layer 216 and themulti-layer stack 230.

As shown in FIG. 7, the two shallow trench isolation portions 242 aredisposed in the semiconductor substrate 210. The switch device 260 isdisposed laterally between the shallow trench isolation portions 242.The two deep trench isolation portions 244 are disposed in thesemiconductor substrate 210 and respectively under the shallow trenchisolation portions 242. The multi-layer stack 230 is disposed laterallybetween the deep trench isolation portions 244. The dielectric layer 262is disposed over the semiconductor substrate 210 and surrounds theswitch device 260.

In conclusion, with the design of the semiconductor structure, thesemiconductor structure has an air void formed in the semiconductorsubstrate. The air void can provide the switch device with betterisolation effect, thereby helping the semiconductor structure reducecapacitance disturbance and raising the switching speed of the switchdevice over the air void.

In addition, the semiconductor structure has multiple filling layershaving different etching rates, which can facilitate the properformation of the air void in the semiconductor structure.

One aspect of the present disclosure provides a semiconductor structure.The semiconductor structure includes a semiconductor substrate, amulti-layer stack, a switch device, and an air void. The multi-layerstack is buried in the semiconductor substrate. The multi-layer stackincludes a first filling layer and a second filling layer under thefirst filling layer, the first filling layer has a first etching rate,the second filling layer has a second etching rate, and the firstetching rate and the second etching rate are different. The switchdevice is disposed over the semiconductor substrate. The air void isformed in the multi-layer stack and under the switch device. The airvoid is surrounded by dielectric filling material.

Another aspect of the present disclosure provides a method formanufacturing a semiconductor structure. The method includes forming amulti-layer stack in the semiconductor substrate, wherein themulti-layer stack has a first filling layer and a second filling layerunder the first filling layer, the first filling layer has a firstetching rate, the second filling layer has a second etching rate, thefirst etching rate and the second etching rate are different, thesemiconductor substrate has two through vias, and two top portions ofthe multi-layer stack are respectively exposed through the two throughvias; recessing the multi-layer stack from the two through vias torespectively form two blind holes in the first filling layer and thesecond filling layer; selectively etching the second filling layer toform a global cavity between the two blind holes; filling the globalcavity and the two blind holes with dielectric filling material so as toform an air void in the multi-layer stack; and forming a switch deviceover the semiconductor substrate, wherein the air void is formed underthe switch device.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods and steps.

What is claimed is:
 1. A semiconductor structure, comprising: asemiconductor substrate; a multi-layer stack buried in the semiconductorsubstrate, wherein the multi-layer stack includes a first filling layerand a second filling layer under the first filling layer, the firstfilling layer has a first etching rate, the second filling layer has asecond etching rate, and the first etching rate and the second etchingrate are different; a switch device disposed over the semiconductorsubstrate; and an air void formed in the multi-layer stack and under theswitch device, wherein the air void is surrounded by dielectric fillingmaterial.
 2. The semiconductor structure of claim 1, wherein the secondetching rate is greater than the first etching rate.
 3. Thesemiconductor structure of claim 1, wherein the multi-layer stackfurther includes: a third filling layer disposed beneath the secondfilling layer, wherein the third filling layer has a third etching ratedifferent from the second etching rate.
 4. The semiconductor structureof claim 3, wherein the second etching rate is greater than the firstetching rate and the third etching rate.
 5. The semiconductor structureof claim 1, wherein the air void is formed in the location of the secondfilling layer.
 6. The semiconductor structure of claim 1, wherein thesemiconductor substrate includes an underlying substrate and anepitaxial semiconductor layer disposed over the underlying substrate,wherein the multi-layer stack is buried in the underlying substrate, andthe epitaxial semiconductor layer is disposed between the multi-layerstack and the switch device.
 7. The semiconductor structure of claim 5,further comprising a well portion embedded in the epitaxialsemiconductor layer, wherein the well portion is disposed under theswitch device and over the air void.
 8. The semiconductor structure ofclaim 5, further comprising: an etch stop layer disposed between theepitaxial semiconductor layer and the multi-layer stack.
 9. Thesemiconductor structure of claim 1, further comprising: two shallowtrench isolation portions disposed in the semiconductor substrate,wherein the switch device is disposed laterally between the shallowtrench isolation portions; and two deep trench isolation portionsdisposed in the semiconductor substrate and respectively under theshallow trench isolation portions, wherein the multi-layer stack isdisposed laterally between the deep trench isolation portions.
 10. Thesemiconductor structure of claim 1, further comprising: a dielectriclayer disposed over the semiconductor substrate and surrounding theswitch device.
 11. A method for manufacturing a semiconductor structure,comprising: forming a multi-layer stack in a semiconductor substrate,wherein the multi-layer stack has a first filling layer and a secondfilling layer under the first filling layer, the first filling layer hasa first etching rate, the second filling layer has a second etchingrate, the first etching rate and the second etching rate are different,the semiconductor substrate has two through vias, and two top portionsof the multi-layer stack are respectively exposed through the twothrough vias; recessing the multi-layer stack from the two through viasto respectively form two blind holes in the first filling layer and thesecond filling layer; selectively etching the second filling layer toform a global cavity between the two blind holes; filling the globalcavity and the two blind holes with dielectric filling material to forman air void in the multi-layer stack; and forming a switch device overthe semiconductor substrate, wherein the air void is formed under theswitch device.
 12. The method of claim 11, further comprising: forming athird filling layer of the multi-layer stack in the semiconductorsubstrate beneath the second filling layer, wherein the third fillinglayer has a third etching rate different from the second etching rate.13. The method of claim 11, wherein the air void is formed in thelocation of the second filling layer and surrounded by dielectricfilling material.
 14. The method of claim 11, further comprising:forming an epitaxial semiconductor layer of the semiconductor substrateover an underlying substrate of the semiconductor substrate, wherein themulti-layer stack is formed in the underlying substrate, and theepitaxial semiconductor layer is formed between the multi-layer stackand the switch device.
 15. The method of claim 14, further comprising:forming a well portion embedded in the epitaxial semiconductor layer,wherein the well portion is formed under the switch device and over theair void.
 16. The method of claim 14, further comprising: forming anetch stop layer between the epitaxial semiconductor layer and themulti-layer stack.
 17. The method of claim 11, further comprising:forming two deep trench isolation portions in the semiconductorsubstrate, wherein the multi-layer stack is formed laterally between thedeep trench isolation portions; and forming two shallow trench isolationportions in the semiconductor substrate and respectively above the deeptrench isolation portions, wherein the switch device is formed laterallybetween the shallow trench isolation portions.
 18. The method of claim11, further comprising: forming a dielectric layer over thesemiconductor substrate and surrounding the switch device.
 19. Themethod of claim 11, further comprising: providing an underlyingsubstrate of the semiconductor substrate before forming a multi-layerstack in the semiconductor substrate, wherein the underlying substratehas the two through vias; and forming two local trenches respectivelyunder the two through vias.
 20. The method of claim 19, furthercomprising: forming a global trench between the two local trenches inthe underlying substrate of the semiconductor substrate; and filling theglobal trench and the two local trenches to form the multi-layer stackin the underlying substrate of the semiconductor substrate.